Proper stabilization of the head is essential for humans to carry out many of the essential activities of daily living. Throughout most of the activities in which we engage the head is held in a stereotyped position with respect to gravity. This helps to maintain the orientation of the head's special sensory receptors in space and regulates the attitude of the head on the trunk as part of overall postural control. Vestibulocollic reflexes (VCRs), which utilize information from sensors of the vestibular labyrinth to generate neck muscle activity to stabilize the head are a critical part of the head stabilization system. They interact with cervicocollic reflexes (CCRs), voluntary and reaction time movements and head-neck biomechanics in controlling head position. To resolve controversies regarding the importance of each of these four mechanisms, our primary goal involves characterizing the 4 mechanisms and determining how they contribute to head stabilization. A second goal is to determine the dynamic and kinematic properties of voluntary head tracking movements. If successful, the proposed resolution of head stabilization and tracking into their component mechanisms will have broad application to many areas of motor control that await similar analysis. To achieve these goals we will pursue three series of experiments in both human and monkey subjects: Exp. 1 will examine the dynamic properties of the open loop VCR and CCR to determine their transfer functions and will characterize the inertial and viscoelastic properties of human and monkey head-neck mechanical plant. Exp. 2 will perform a similar analysis of the closed loop VCR, where the head is free to move in response to body rotation. Exp. 3 will analyze the dynamic and kinematic properties of voluntary head tracking using electromyographic and fluoroscopic recording to obtain data to test our detailed biomechanical models of the head-neck system. Experimental results will be interpreted using two models. The first is a dynamic model which starts with well tested vestibuloocular reflex models and adds biomechanical properties and multiple rotation axes that characterize the head movement system. It will incorporate elements corresponding to known physiology of labyrinthine receptors and reflex pathways and will attempt to show how position, velocity and acceleration information, embedded in firing patterns of regular and irregular peripheral afferents, drives neck muscles to maintain stability of the head in space. The second is a detailed biomechanical model of the human head-neck system that quantifies the actions of all joints, muscles and passive mechanics and allows prediction of appropriate patterns of muscle activity to stabilize the head in the face of angular and linear perturbations.